This invention relates to a transistor amplifier and, more particularly, to a transistor amplifier comprised of a field effect transistor having triode-type dynamic characteristics wherein the DC bias current flowing therethrough is stabilized, notwithstanding voltage fluctuations in the power supply.
The use of a field effect transistor (FET) having triode-type dynamic characteristics offers many advantages which cannot be obtained from conventional FET's which have pentode-type dynamic characteristics. For example, harmonic distortion is reduced and switching characteristics are improved for the FET having triode-type dynamic characteristics. Also, this type of FET has a greatly improved linearity function. Consequently, the FET having triode-type dynamic characteristics finds ready application in high fidelity audio equipment, especially as an audio output amplifier.
One problem of such an FET having triode-type dynamic characteristics is that the DC bias current which flows through the drain-source circuit will be subjected to variations if the operating potential applied to the FET by the voltage supply exhibits fluctuations. In the application of audio amplifier, large currents are often required, resulting in significant variations in the voltage supply. The consequential bias current variations can result in undesirable distortion in the amplified output of the FET.
One solution to this problem has been proposed in U.S. Pat. application Ser. No. 508,836, filed on Sept. 24, 1974, and assigned to the same assignee as the instant invention. As previously proposed, a bias circuit is connected to the gate electrode of the FET for applying a suitable bias voltage thereto. This bias circuit includes a DC compensating circuit which serves to vary the FET gate bias in response to fluctuations in the operating potential supplied by a power supply. In particular, the change in the gate bias is intended to be directly proportional to the change in operating potential with a proportionality factor related to the amplification constant of the FET.
Although this arrangement offers satisfactory results in most instances, it is desirable to simplify the requisite circuitry. Also, it is preferred to have the change in the gate bias more closely related to the fluctuations in the FET power supply. Accordingly, another proposal to stabilize the DC bias current of the FET is described is copending Application Ser. No. 577,678. The present invention is directed to a still further arrangement for achieving FET bias current stabilization.